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A generic tight-binding model for monolayer, bilayer and bulk MoS2  [PDF]
Ferdows Zahid,Lei Liu,Yu Zhu,Jian Wang,Hong Guo
Physics , 2013,
Abstract: Molybdenum disulfide (MoS2) is a layered semiconductor which has become very important recently as an emerging electronic device material. Being an intrinsic semiconductor the two-dimensional MoS2 has major advantages as the channel material in field-effect transistors. In this work we determine the electronic structure of MoS2 with the highly accurate screened hybrid functional within the density functional theory (DFT) including the spin-orbit coupling. Using the DFT electronic structures as target, we have developed a single generic tight-binding (TB) model that accurately produces the electronic structures for three different forms of MoS2 - bulk, bilayer and monolayer. Our TB model is based on the Slater-Koster method with non-orthogonal sp3d5 orbitals, nearest-neighbor interactions and spin-orbit coupling. The TB model is useful for atomistic modeling of quantum transport in MoS2 based electronic devices.
Intense femtosecond photoexcitation of bulk and monolayer MoS2  [PDF]
I. Paradisanos,E. Kymakis,C. Fotakis,G. Kioseoglou,E. Stratakis
Physics , 2015, DOI: 10.1063/1.4891679
Abstract: The effect of femtosecond laser irradiation on bulk and single-layer MoS2 on silicon oxide is studied. Optical, Field Emission Scanning Electron Microscopy (FESEM) and Raman microscopies were used to quantify the damage. The intensity of A1g and E2g1 vibrational modes was recorded as a function of the number of irradiation pulses. The observed behavior was attributed to laser-induced bond breaking and subsequent atoms removal due to electronic excitations. The single-pulse optical damage threshold was determined for the monolayer and bulk under 800nm and 1030nm pulsed laser irradiation and the role of two-photon versus one photon absorption effects is discussed.
Strain-induced magnetism in MoS2 monolayer with defects  [PDF]
Peng Tao,Huaihong Guo,Teng Yang,Zhidong Zhang
Physics , 2013, DOI: 10.1063/1.4864015
Abstract: The strain-induced magnetism is observed in single-layer MoS2 with atomic single vacancies from density functional calculations. Calculated magnetic moment is no less than 2muB per vacancy defect. The straininduced band gap closure is concurrent with the occurrence of the magnetism. Possible physical mechanism of the emergence of strain-induced magnetism is illustrated. We also demonstrate the possibility to test the predicted magnetism in experiment. Our study may provide an opportunity for the design of new type of memory-switching or logic devices by using earth-rich nonmagnetic materials MoS2.
Strain tuning of optical emission energy and polarization in monolayer and bilayer MoS2  [PDF]
G. Wang,C. R. Zhu,B. L. Liu,X. Marie,Q. X. Feng,X. X. Wu,H. Fan,P. H. Tan,T. Amand,B. Urbaszek
Physics , 2013, DOI: 10.1103/PhysRevB.88.121301
Abstract: We use micro-Raman and photoluminescence (PL) spectroscopy at 300K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS2 on a flexible substrate. The initially degenerate E^1_{2g} Raman mode is split into a doublet as a direct consequence of the strain applied to MoS2 through Van der Waals coupling at the sample-substrate interface. We observe a strong shift of the direct band gap of 48meV/(% of strain) for the monolayer and 46meV/% for the bilayer, whose indirect gap shifts by 86meV/%. We find a strong decrease of the PL polarization linked to optical valley initialization for both monolayer and bilayer samples, indicating that scattering to the spin-degenerate Gamma valley plays a key role.
Strain effects on the spin-orbit induced band structure splittings in monolayer MoS2 and graphene  [PDF]
Tawinan Cheiwchanchamnangij,Walter R. L. Lambrecht,Yang Song,Hanan Dery
Physics , 2013, DOI: 10.1103/PhysRevB.88.155404
Abstract: The strain effects on the spin-orbit induced splitting of the valence band maximum and conduction band minimum in monolayer MoS2 and the gap in graphene are calculated using first-principles calculations. The dependence of these splittings on the various symmetry types of strain is described by means of an effective Hamiltonian based on the method of invariants and the parameters in the model are extracted by fitting to the theory. These splittings are related to acoustic phonon deformation potentials, or electron-phonon coupling matrix elements which enter the spin-dependent scattering theory of conduction in these materials.
Atomistic simulation of the electronic states of adatoms in monolayer MoS2  [PDF]
Jiwon Chang,Stefano Larentis,Emanuel Tutuc,Leonard F. Register,Sanjay K. Banerjee
Physics , 2013, DOI: 10.1063/1.4870767
Abstract: Using an ab initio density functional theory (DFT) based electronic structure method, we study the effects of adatoms on the electronic properties of monolayer transition metal dichalcogenide (TMD) Molybdenum-disulfide (MoS2). We consider the 1st (Li, Na, K) and 7th (F, Cl, Br) column atoms and metals (Sc, Ti, Ta, Mo, Pd, Pt, Ag, Au). Three high symmetry sites for the adatom on the surface of monolayer MoS2 are examined as starting points to search for the most energetically stable configuration for each adatom-monolayer MoS2 system, as well as the type of associated bonding. For the most stable adatom positions, we characterize the emergence of adatom-induced electronic states including any dopant states.
Temperature Renormalization of Optical Spectra of Monolayer MoS2  [PDF]
Ryan Soklaski,Yufeng Liang,Changjian Zhang,Haining Wang,Farhan Rana,Li Yang
Physics , 2014, DOI: 10.1063/1.4878098
Abstract: Newly measured optical absorption and photoluminescence spectra reveal substantial frequency shifts of both exciton and trion peaks as monolayer MoS2 is cooled from 363 K to 4 K. First-principles simulations using the GW-Bethe-Salpeter Equation approach satisfactorily reproduce these frequency shifts by incorporating many-electron interactions and the thermal expansion of the in-plane lattice constant. Studying these temperature effects in monolayer MoS2 is crucial for rectifying the results of room-temperature experiments with the previous predictions of zero-temperature-limit simulations. Moreover, we estimate that the thermal expansion coefficient of monolayer MoS2 is around 25% less than that of bulk counterpart by tracking the frequency shifts of the exciton or trion peaks in optical spectra. This may serve as a convenient way to estimate thermal expansion coefficients of general two-dimensional chalcogenides.
Gas adsorption on MoS2 monolayer from first-principles calculations  [PDF]
Shijun Zhao,Jianming Xue,Wei Kang
Physics , 2013, DOI: 10.1016/j.cplett.2014.01.043
Abstract: First-principles calculations within density functional theory (DFT) have been carried out to investigate the adsorption of various gas molecules including CO, CO2, NH3, NO and NO2 on MoS2 monolayer in order to fully exploit the gas sensing capabilities of MoS2. By including van der Waals (vdW) interactions between gas molecules and MoS2, we find that only NO and NO2 can bind strongly to MoS2 sheet with large adsorption energies, which is in line with experimental observations. The charge transfer and the variation of electronic structures are discussed in view of the density of states and molecular orbitals of the gas molecules. Our results thus provide a theoretical basis for the potential applications of MoS2 monolayer in gas sensing and give an explanation for recent experimental findings.
Enhanced absorption of monolayer MoS2 with resonant back reflector  [PDF]
Jiang-Tao Liu,Tong-Biao Wang,Xiao-Jing Li,Nian-Hua Liu
Physics , 2014, DOI: 10.1063/1.4878700
Abstract: By extracting the permittivity of monolayer MoS2 from experiments, the optical absorption of monolayer MoS2 prepared on top of one-dimensional photonic crystal (1DPC) or metal films is investigated theoretically. The 1DPC and metal films act as resonant back reflectors that can enhance absorption of monolayer MoS2 substantially over a broad spectral range due to the Fabry-Perot cavity effect. The absorption of monolayer MoS2 can also be tuned by varying either the distance between the monolayer MoS2 and the back reflector or the thickness of the cover layers.
Temperature-dependent Phonon Shifts in Monolayer MoS2  [PDF]
Nicholas Lanzillo,A. Glen Birdwell,Matin Amani,Frank J. Crowne,Pankaj B. Shah,Sina Najmaei,Zheng Liu,Pulickel M. Ajayan,Jun Lou,Madan Dubey,Saroj K. Nayak,Terrance P. O'Regan
Physics , 2013, DOI: 10.1063/1.4819337
Abstract: We present a combined experimental and computational study of two-dimensional molybdenum disulfde (MoS2) and the effect of temperature on the frequency shifts of the Raman-active E2g and A1g modes in the monolayer. While both peaks show an expected red-shift with increasing temperature, the frequency shift is larger for the A1g more than for the E2g mode. This is in contrast to previously reported bulk behavior, in which the E2g mode shows a larger frequency shift with temperature. The temperature dependence of these phonon shifts is attributed to the anharmonic contributions to the ionic interaction potential in the two-dimensional system.
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